Electrical devices built from gallium arsenic (GaAs) offer many advantages over conventionally built silicon semiconductor devices. In particular, gallium arsenide devices operate at higher speeds than silicon devices. This is due to the fact that electrons have a higher mobility in gallium arsenide than in silicon. However, the speed advantage inherent in gallium arsenide cannot be realized unless the electrical signals produced by the individual devices can be transmitted to other devices. Gallium arsenide devices, like silicon devices, must be connected to other devices without impairing the signal transmission between devices. Therefore, transmission of signals to and from the devices requires high quality contacts. This is a particular problem for gallium arsenide because contacts are not as easily made to gallium arsenide devices as they are to silicon devices. Therefore, unless high quality contacts can be made to gallium arsenide, the speed advantage of gallium arsenide over silicon will not be realized.
The electrical quality of the contact to the gallium arsenide device is measured by the resistance (called contact resistance) between the gallium arsenide semiconductor and the contact metallization. If the current flow through the contact is linear with the applied voltage across the contact and the resistance (V=IR) is low, the contact is said to be a good ohmic contact. Typically, the various gallium arsenide devices are interconnected during circuit fabrication by aluminum alloy wiring, although other types of metals can be used. However, merely depositing the aluminum (or most other metals) directly onto the gallium arsenide creates a Schottky diode rather than an ohmic contact. This diode can be made into an ohmic contact to n-type GaAs by doping the GaAs with an n-type dopant. Specifically, the dopant concentration in the GaAs must be in excess of approximately 1E19 per cubic centimeter to create an ohmic contact. This level of doping creates enough tunnelling current through the diode potential barrier that the diode behaves electrically like an ohmic contact. The problem with this solution to GaAs n-type contacts is that conventional doping and annealing techniques of GaAs are not sufficient to dope GaAs to such high levels. Therefore, in order to create an ohmic contact to GaAs, the metallurgy itself must supply the necessary dopants.
Prior art attempts to solve the problem of GaAs contact resistance have generally included a germanium-gold (Ge-Au) alloy layer interposed between the n-type gallium arsenide and the wiring metallurgy. This multi-element alloy is employed to facilitate the incorporation of a dopant into the GaAs lattice. A commonly used ohmic contact to GaAs is based on the eutectic germanium-gold (Ge-Au) alloy (88 weight % Au), in conjunction with nickel (Ni). The compound is typically annealed above the eutectic temperature of approximately 360 degrees Celsius which results in the alloy being melted. The gold is highly reactive with Ga from the GaAs lattice and forms Au-Ga compounds during the annealing process. This leaves Ga vacancies in the GaAs lattice which are occupied by Ge, an n-type dopant. This n-type dopant contributes to the tunnelling current and helps form an ohmic contact even though the doping concentration without the contact metallurgy is less than 1E19/cubic centimeter. The problem with this solution to the contact problem is that, in addition to having low resistance, contacts must be thermally stable. Device processing after the contact formation includes multilevel interconnect processing and packaging. These steps can expose the GaAs device to temperatures of approximately 400 degrees Celcius for periods of a few minutes to several hours. Heating the eutectic based Ge-Au-Ni contacts to 400 degrees after contact formation results in local melting of the contact which raises the contact resistance. In addition to the contact resistance being raised generally, the variance in contact resistance between devices on a chip is also raised. This is because the melting of the contact metallurgy does not occur uniformly across the chip. Therefore, the eutectic based Ge-Au-Ni contact is not a solution to the GaAs contact resistance problem because the processes after the contact formation degrade the Ge-Au-Ni compound contact resistance. Another prior art attempt to produce a reliable contact structure to gallium arsenide comprises a plurality of layers of different metals. Specifically, the contact structure includes a first layer of nickel covered by a thin layer of gold followed by a Ge-Ni layer and a tungsten (W) layer. The gold content in the annealed NiGe(Au)W compound is low compared to the eutectic Ge-Au-Ni compound. The NiGe(Au)W compound is more thermally stable than the eutectic Ge-Au-Ni compound and it still retains enhanced contact resistance. The tungsten is added in order to reduce the sheet resistance of the low Au content alloy. The resulting contact structure is characterized by a low contact resistance which has high thermal stability, smooth morphology and a uniform metal to gallium arsenide interface. These characteristics are essential for providing the improved reliability of the metal to gallium arsenide contact structure. The problem with this particular contact structure, however, is that tungsten has a very high melting point. Therefore, evaporating tungsten onto gallium arsenide is incompatible with VLSI processing of high performance gallium arsenide devices. Furthermore, conventional sputtering of the tungsten onto the identical nickel/germanium/gold layered structure results in contacts with poor contact resistance. Sputtering the tungsten, rather than evaporating it, defeats the purpose of the contact structure.